Daemons
A daemon is a long-running (runs continuously as long as the system is running) background process that answers requests for services. Daemons don’t have a direct user interface. The term originated with Unix, but most operating systems use daemons in some form or another. In Unix, the names of daemons conventionally end in “d”. Some examples include inetd, httpd, nfsd, sshd, named, and lpd.